The invention relates to a method for controlling compressor operation under extreme power supply conditions of line frequency and voltage.
Compressors are utilized in different refrigerant vapor compression applications, including refrigeration, air conditioning, heat pumps, etc. Typically, these compressors include an electric motor driving a compressor pump unit. The compressor pump unit compresses the refrigerant and delivers it into the refrigerant system.
For the compressor to operate property the compressor operating speed must fall within a certain range. The compressor speed is a function of the line frequency, voltage, and the load on the compressor. This can be explained as follows. The speed of the electrical motor used in typical refrigerant compressing applications is proportional to line frequency minus motor slip. The motor slip increases as supplied voltage is decreased or a compressor load is increased. Therefore, the compressor motor speed will decrease if the frequency decreases, if the load increases, or if the voltage decreases. Compressors are not designed to operate properly below a certain speed.
For example, scroll compressors may have a feature called radial compliance in which centrifugal force keeps an orbiting scroll pressed against fixed scroll in a radial direction. If the scroll compressor operates below a certain speed, the radial compliance can be lost, because centrifugal force keeping the scrolls together drops below the minimum acceptable value. Further, if an oil pump is employed, oil will not be delivered to lubricate scroll compressor components below a certain operating speed. These are undesirable effects of operating scroll compressor at reduced speed.
The overall force acting on a main scroll compressor bearing consists of two components. The first component of the force is proportional to compressor load; and the second component, caused by rotating shaft counterweights, is proportional to speed squared. Thus, as speed increases to an undesirably high level at a given compressor load, the overall force acting on the bearing can become excessively high, which is undesirable. Then, to decrease the force acting on the bearing the compressor speed must be decreased or compressor load decreased.
The operation under extreme conditions of line frequency and voltage and resultant operating speed excursions are especially common where the electric power is supplied by a generator set, since in this application frequency and voltage often fluctuate extensively, especially on start up.